Audio amplifier circuits for radio transmitters



Sept. 13, 1949. J. H. PRATT AUDIO AMPLIFIER CIRCUITS .FOR RADIO TRANSMITTERS Filed Oct. 28, 1.944

' Jef/N H. :D BY.

Saz.

INVENTOR.

A T r ATTORNEY Patented Sept. 13, 1949 AUDIO AMPLIFIER CIRCUITS FOR RADIO TRANSMITTERS John H. Pratt, Montreal, Quebec, Canada, as-

signor, by mesne assignments, to Radio Corpo ration of America, New York, N. Y., a corporation of Delaware Application October 28, 1944, Serial No. 560,761 In Canada June 6, 1944 This invention relates to audio or speech amplifier'circuits for use in radio transmitting apparatus, and more particularly to such amplifiers utilizing inverse feedback for obtaining low distortion and reduced noise in such amplifier circuits.

In one specific aspect the invention is embodied in the audio frequency section of a transmitter which is adapted for various types of transmission, to wit: radio telephone (RT), continuous wave (CW), and modulated continuous wave (MCW).

One of the objects of the invention is to improve generally the operation of audio amplifiers by the utilization of several inverse feed-back circuits.

A more specific object of the invention is to lower the apparent plate resistance of the driver tube so as to enable it to impress an undistorted wave on the grids of the push-pull modulator tubes which present a non-linear resistance.

A further object of the invention is to modify the frequency response characteristic in a manner found desirable to increase the intelligibility of speech in noisy locations such as in aircraft.

A further object is to obtain the frequency response characteristic referred to without the use of filters of the type employing inductance coils, thus savingspace and weight.

A still further object is to provide a regenerative feedback circuit for the driver stage so that it will oscillate at a predetermined frequency, as for example, 1000 cycles, for tone modulation of the radio frequency carrier on MCW and to supply side-tone on CW, said oscillations to be stable in both frequency and amplitude.

Other objects and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawing in which the single figure is a schematic circuit diagram of the audio frequency section of the transmitter.

Referring to the figure, I and 2 are electron discharge tubes which function respectively as first and second audio amplifiers, and may be of the type known as 12SJ7 which is a pentode provided with an indirectly heated cathode. The electron discharge tube designated 3 is a driver for the push-pull modulator tubes designatedl 4 and 5. The driver tube may be a heater type beam tube of the type known as 1625 and the modulator tubes may be triodes of the directly heated type known as 811.

The input of the first audio amplifier tube I is coupled through a transformer TI to a source of audio frequency signals or speech currents from a microphone, not shown. The amplitude vthe signal grid of the tube.

of the signals applied tothe signal grid of the first amplifier is adjusted by means of a potentiometer 6 across the secondary of the transformer, the adjustable tap 1 being connected to Cathode resistor 8 serves to provide grid bias for tube I in the usual manner.

The output of first audio amplifier I is resistance-capacity coupled in the conventional manner to the input of the second audio amplifier 2, the coupling network being constituted by 9, the plate resistor of tube I, coupling capacitor I0 and the grid resistor I I for tube 2. Likewise, with the switch S in the position shown for RT or radio telephone transmission, the output of the second audio amplifier 2 is resistance-capacity coupled to the input of the driver tube 3 operating as a class A amplifier, plate resistor I2, coupling condenser I3 and grid resistor I4 constituting the coupling network. The output of the driver tube 3 is coupled to the push-pull modulator tubes 4 and 5, operating as class B amplifiers through atransformer T2 which is provided with two secondaries I5 and I5 connected respectively, to the grids of the modulator tubes 4 and 5. The output of the latter tubes are coupledto the power amplifier of the transmitter through the modulation transformer T3 which is provided with secondary windings I'I and I8 connected respectively to the plate and screen of the power amplifier.

An amplifier such as driver tube 3, feeding a highly non-linear load resistance, such as the grid of a class B modulator, must have a low internal impedance. For the purpose of lowering the apparent plate resistance of the driver tube there is provided an inverse feedback circuit I9 from the output of tube 3 to the input of tube 2. This circuit includes resistors 20, 2| and 22 which form a voltage divider to feed a portion of the voltage appearing at the -plate of driver tube 3 to the cathode 23 of tube 2. This voltage is in inverse phase relationship with the input voltage applied to the signal grid of tube 2.

It is desirable that the total resistance of the voltagedivider 20, 2| and 22 be kept high in order that the audio frequency power dissipated in the voltage divider be small. This is accomplished by using the two resistors 2| and 22 in the cathode circuit of tube 2 and returning the grid resistor I I to their junction. By this means the total resistance in the cathode circuit is made, high enough to give the desired value of feedback voltage with a high total voltage-divider resistance, and at the same time the correct value of bias is applied to the grid of the tube.

Shunted across resistor 20 are the series-con- .Ileted resistor 24 and capacitor 25 which serve to increase the feedback at high frequenciesl above about 3000 cycles per second and'thereby help to attenuatefthem. Resistor 24"is*l:ow in' Valuelcompared to''the'reactanc'e df capacitorziin'the audio frequency range. Its purpose is to prevent oscillations by limiting the feedback at very high fre'- quencies where the phase of thelfeedbackvoltage A y becomes positive.

A second feedback loop to furtherV modify the frequency response is employedstoagivea "sharp low-frequency cut-off belofwapproximately 1000 cycles.V It will be noted thatethe screen lvoltage for the first audio tube l is obtained through resistors 25 and 21 from theiplateof driver tube 3. Capacitors Y28 and 29 lter frequencies above plates and screens of tubes I,y 2 and 3 and to the screen of the power amplifier. The resistor-conidenserfcombination M'jrlsfforfthe purpose of :filteringt'he'voltage suppl-y to .the plate of tube l and to the plate and screen of tube 2. Further iiltering of the plate voltage supply for tube l .isQbtainedbymeans of the resistance-condenser combination '153, 44 and further filtering of thev Y screen voltage supply for tube 2 is obtained by 45,00.'" A-second:or high voltage source repre- Y 'imeans fofthe resistance-condenser combination sentefclby -{BZ serves to apply suitablerpotentials "toi'the plates of the modulator tubes 4, 5

j rand-.to the .plateof the power amplifier.

1000 cycles from this lead, but lower frequenciesY v are applied to the screen grid of tube v'l in such Dhaseas to oppose the effect of the input signal f uniche 'contrai@franchis/aube. maefiower .the frequency, the greater fis ithe' negative ffeedback, so the effective 'gain fof the farnplier A"decreases rapidly 'with f'frequenoy. y

'The gain vo'f va feedback A'amplifier -is zproportional fto "1 fl-B 1in which' is `lthe 'gain-of the amplifier without feedback and B is the '-'fraction -of the output '-voltagefappliedto fthe input; V'Both A andB are vector-'quantities SinceAbeccmes small an'd 'hasea@large phase shift at `Iverylow and very hi-g'h ffrequences, ifeedback is Vnot effective in reducinggain atthese'ifrequencies' However, -there f'is fno `telfdienoy lfor the gain to frise because ,the portions of lthe amplifier fouts'i'de 'the feedback loops Aare so-designed that-*thegain'of Athese portions drops -olf outside the Vband Aof den sired frequencies, 1 effectively *maskingy any vregen" Y erative effects` -occurring Lin the *feedback loops.

vWith fthe transrn'itter -nperating on yMCW` or fCW, the -switchfS'willbefmoved'tolits appropriate contact. Z'For 'these -operations the driver V'stage 3 is entirely =disconnected 'Tfr'orn "the 1tweaudio amplifier tubes i and "2 and 'is made `to oscill'ate `by connecting its rgidtoea rfrequency 'responsive a switch I39 wliich'is actuated 'b yan'd is under the 'control 4'of the-keying 1relay, not lshown. On CW this networkconsists -of Aresistors *31, I3.2,(33

Y and condenser f34'an'd-oniMCWitconsists ofreandto obtain oscillations (if :substanti'allysinevwave formi, yit *is Jnecessary Ltofuse negative feedyback in this stage. The negativefeedback cannot be obtainedthxoughithe "IU'OD ,'.Whih Aincludes vthe second 'amplifier- .tube 2 loecause'fit :is 'discon- `nectecL'so another"negative'feeiiback loop .is provided by returningthje cathode'oftube 3 Lthrough an Aau-xiliary'w'indi-ng '39 `on the lzlrivertransforiner T2. Thejfeedback 'obtainedjin' this manner "is substantially =of `the"voltage,type"and" -aiiz'i's tothe the source impedance df'ith'efirivejstage when lon telephone operation. LAr'efsis'tlaL-ncej-capacity 'corn- Ybinati'on' 4i) is- Afconnect'ed *between the auxiliary Winding 39 and; ground-@tofprovide-:griddoias iforthe driver tube. Y

A `first, or "low voltage source represented by L1-'BI serves to apply suitable potetialsto the .'It'i will be clear from the above that I have provided an audio amplifier in which negative or 'inverse feedback is employed to improve the oper- Y ation in respectvto noise and distortion,w andalso ftd modify the frequency-response -fcharacteristic in adesired manner. Also the audio v:amplifier "is capable-of supplyingstablefselfoscillations gror thepurpose of modulating aradi'ofrequency .fsi'g- :and 4pi-"oviding 'sid'etoneito @the telephone :re-

ceiver of a person operating the telegraph fkey. 'It will fb'e lclear Aalso that Ytheseresultsfare ioblp'loying inductance coils @so lthat the lcir'cuits fdis- `':losedand described herein faref'simplenless ex- Y pensive fand 'have f ewer components# While I have shown a particular embodiment of Vmyinvention, 'it will, of course, be understood various modifications -Inayibe1made'therein without departing from the spirit and scope off-my invention I'a's set forth'fin Ethe 'appended claims,

"and I contemplate 'bythe' appended fcl'aimsrfto cover'anysuch modifications as fall within'the ftrue spirit and 'scopeio'f my -invention.`

t'1. 'Infcombinatiorr an `audio frequ'ency ampliillcatitinY fstage. includin'g 'an felectrontlischarge '.cathode',-input and ioutp'ut circuits'and anunvoltage divider connected 'across the -secondary Y winding i6 of the-driverftransformer 'T2 through bypassed resistance 'return connected "tossa'id `fcatlioile;`1a 'source of signals-connectedto vthe Y cuit connected betweenthe Tanode 4'ofth'efdriver `=input1 Vldisclfarge anodel l'co'ur'iled toV "the "output Jcircuit, Y a push-pull mcdulatorfs'tage A:transformer'cou- :pled 4'for' driving `:by fth'e driver stage; and means rcuit,"a driver -Stagefincludling ali-'electronfor reducing `the apparent lectron-discharge resistance of the driver 4'stage to -cause' Ait gto'V impress an undis'torted Waveren'"th'efmodulatorl stage,

zsaid'fmeans comprising'an'inver'se feedback cirstage and the cathode endpflsaidjunbypassed're- 7sistancaaand 'said feedback'fcircuit including a resistance which 'is lshuntedby aseri/esv-connected "resistance and condenser *to increase feedback jj actionat the higher frequencies a'ndtfov limitjthe Y'feedback at thehighest frequencies Where the signals fedback become regenerative. .K

Y2. In fcombination: cascaded l"virstvand Vsecond f :audio frequency vamplifier "stages each including fan 'electron-discharge: 'tube ,unit of 'thegmulti- K grid f type and input output. circuitsfa source lof signals connected tothefinput "circuit of the `irst tube unit;l a r'nodula'torl driver Ystage Aincluding anelectromdischarge' tubeunit havin'gra jp1ate said driver tube Vunit being coupledA tothe output circuit ofthe second tube unit; aV first inverse 'feedback `path `comprisingI a Vflrst-res'istor .connected between the 'plate of 'the driver tube junit and the cathode of the Ysec'ond'tube unit, second'and third 'resistors connected inthe return 'circuit of said cathode, 'andlaconnectionifrom be'tween saidrsecond jand third resistors to the" control grid of the second tube unit; and a second inverse feedback circuit connected between the plate of the driver tube unit and a grid of the first tube unit other than the signal control grid; said second feedback circuit including a pair of series-connected resistances, 'the resistor adjacent the grid being bypassed to ground at each end thereof by a condenser to provide the amplifier stages with a sharp low-frequency cut-ofi'.

3. In combination; Icascaded first and second audio frequency amplifier stages each including an electron-discharge tube unit having a control grid, a screen grid and input and output circuits; a source of signals connected to the input circuit of the first tube unit; a driver stage including an electron-discharge tube unit having a plate and cath-ode, said driver tube unit being -coupled to the output circuit -of the second tube unit; va .push-pull modulator stage including a .pair of electron-discharge tube units coupled for drivcathode circuit of the driver unit and in coupled relation with one of said secondary windings.

4. In combination: cascaded rst and second vaudio frequency amplifier stages each including an electron discharge tube having a contr-o1 grid and a screen grid; a source of signals connected to the input of the first tube; a driver stage including an electron discharge tube coupled to the output of the second tube; a push-pull modulator stage including a pair of electron-discharge tubes coupled to the output of the driver tube through a transformer having a secondary for each modulator tube; a first inverse feedback circuit connected between the plate of .the driver tube and the cathode of the second tube; a second inverse feedback circuit connected between the plate of the driver tube and the screen grid of the first tube; and a third inverse feedback circuit comprising an auxiliary winding included in the cathode circuit of the driver tube and in coupled relation -with one of said transformer secondaries.

5. An audio frequency amplifier comprising at least three cascade-connected vacuum tube stages, the tubes of the first two stages having a control grid and a screen grid, and that of the Y third stage being of the beam power type having a high plate resistance; an inverse feedback circuit including a resistance connected between the plate of the third stage and the cathode of the second stage for reducing the apparent plate resistance of the third stage; a second inverse feedback circuit including series resistance and shunt condensers connected between the .plate of the third stage and the screen grid of the first stage; said second circuit constituting a low-pass filter for effecting increased negative feedback at frequencies below approximately 1,000 cycles; and a series path comprising a capacitor and a resistor connected in shunt -across the resistan-ce included in Ithe first feedback circuit for effecting increased negative feedback at frequencies above `approximately 3,000 cycles.

6. In an audio frequency amplification system for amplifying intelligence conveying signals having a noisy background: a plurality of cascaded electron-discharge tube amplification stages, a first inverse feedback circuit having low-pass :characteristics connected for preferentially returning signals having frequencies of not more than about 1000 cycles .per second to establish a low cut'off limit for the amplification gain; `and a second inverse feedback circuit connected between different stages for preferentially returning signals having frequencies of about 3000 or more cycles per second to attenuate the gain at the high frequency end for providing increased intelligibility of the amplied signals with respect to the noisy background, the second feedback circuit including a resistance feedback path and a capacitance feedback path in parallel to the resistance .path for preferentially passing signals of the higher frequencies, and the capacitance feedback .path includes irnpedance elements for limiting the preferential capacitive feedback at the highest frequencies ywhere the signals fed back are shifted in phase tending to make the feedback regenerative.

7. In combination: an audio frequency amplifier stage including an input circuit, an output circuit, and an electron-discharge unit `connected between the input and output circuits, said unit including :an electron-discharge cathode and an unbypassed resistance return connected to said cathode; :a source of signals connected to the input circuit of said amplifier stage; an additional stage having an output circuit and an electron discharge unit including an anode coupled to the output circuit of said amplifier stage; and means for reducing the distortion produced by said stages, said means comprising an inverse feedback circuit connected between the anode of the additional stage and the cathode end of said unbypassed resistance return, and said feedback circuit including a resistance which is shunted by a series-connected resistance and condenser to effect a high frequency cut-off by increasing lthe feedback action at the higher frequencies and limiting the feedback at the highest frequencies where the signals fed back become regenerative.

JOHN I-I. PRATT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,123,241 Harley July 12, 1938 2,162,986 Wiebusch June 20, 1939 2,186,571 Beale Jan. 9, 1940 2,218,902 Blumlein Oct. 22, 1940 2,241,534 Blumlein May 13, 1941 2,264,715 Rohr Dec. 2, 1941 2,266,168 Crabtree Dec. 16, 1941 2,273,997 Rubin Feb. 24, 1942 2,288,600 Arndt July 7, 1942 2,289,752 Bruch July 14, 1942 2,315,040 Bode Mar. 30, 1943 2,342,822 Rumpel Feb. 29, 1944 2,359,504 Baldwin Oct, 3, 1944 2,439,844 Davis Apr. 20, 1948 OTHER REFERENCES Article in the Proceedings of the I. R. E., March 1939, entitled Control of the effective internal impedance of amplifiers by means of feedback, by H. F. Mayer. 

